1. Field of the Invention
The present invention relates to a technique for preventing rotation error of an image bearing body in an image forming apparatus, or a technique for preventing color drift when transferring a toner image for each color formed by each of a plurality of image bearing bodies onto a transfer medium such as an intermediate transfer body or the like.
2. Description of the Related Art
A color image forming apparatus comprises image bearing bodies for original colors of yellow (Y), magenta (M), cyan (C) and black (K), in which a toner image for each color formed on each image bearing body is superposed on an intermediate transfer body, and a toner image with four colors superposed is transferred onto a transfer paper to form a color image.
Each image bearing body forms an image forming section comprising a charging device, a developing device, a transferring device, a cleaning device and the like, and each image forming section for each color and the intermediate transfer body form a process cartridge as a unit. The process cartridge is withdrawable from the body of the image forming apparatus, and each image bearing body, developing device or the like can be attached to or detached from the process cartridge in a state in which the process cartridge is withdrawn.
The toner image for each color formed in each image forming section is transferred onto the rotating intermediate transfer body by the transfer device in order to form a composite color image. A toner image needs to be superposed with a previous image corresponding thereto for forming a color image with no color drift.
For this purpose, linear velocities of the image bearing body and a transfer medium such as the intermediate transfer body are required to correspond to one another with high accuracy. Difference between the linear velocities would cause color drift or image drift.
Each image bearing body is provided with a drive device to be driven. The drive device transmits rotation which was slowed down through a plurality of gears from a main motor to a rotary shaft. The rotary shaft is capable of inserting through the cylindrical image bearing body from one side, and is supported by a bearing provided on the opposite side to rotate the image bearing body.
As described above, the image bearing body receives the rotation from the main motor through a train of gears comprising a plurality of gears, so that high accuracy is required for each gear, however, slight rotation error occurs due to accumulated tolerance of the gears. Rotation error occurs even to the intermediate bearing body which is driven in the same manner as the image bearing body. Thus, rotation error would occur even when adjusting linear velocities of the image bearing body and the intermediate transfer body, thereby causing image drift.
An image forming apparatus disclosed in JP-Tokukai-2000-112194A has a coupling shape in which a bearing of a large gear is engaged with an engaging part of an engaging member of an image bearing body, and transmission of rotation from a rotary shaft to the image bearing body is performed by directly coupling a large gear to the rotary shaft. In such configuration, transmission of the rotation from the main motor to the image bearing body is slowed down through the large gear, intending to improve rotation error by using less number of gears to reduce the influence of accumulated tolerance.
Although rotation error can be improved, it does not become zero. In a color image forming apparatus, rotation error occurs even in the intermediate transfer body, so that the above described configuration would fail to prevent rotation error which occurs while transferring an image onto the intermediate transfer body.